Digital microphones are known and are increasingly being used in some applications, such as for portable electronic devices. FIG. 1 illustrates a conventional arrangement of a digital microphone in use. The digital microphone 101 comprises a transducer such as a MEMS microphone 102 with an associated amplifier 103, e.g. a low noise amplifier, and conversion block 104 co-located with the transducer 102. In some instances the amplifier 103 and conversion block 104 may be formed as an integrated circuit on the same semiconductor die as the transducer 102, but in other arrangements the amplifier 103 and conversion block 104 may be formed on a separate chip 105 to the transducer 102 but packaged together.
The conversion block comprises an analogue-to-digital converter 106 to convert the amplified microphone signal output from the amplifier 103 into a digital signal. In some instances the output of the ADC 106 may be used directly as the output DATA of the digital microphone 101, but in some instances the digital signal produced by the ADC 106 may be re-coded or modulated by a coder 107 into a particular data format, such as an oversampled PDM data stream. In some instances the function of the ADC 106 and coder 107 may be combined so that the conversion block 104 is a 1 bit PDM ADC.
In use the digital microphone will be coupled to an audio circuit 108, such as an audio codec. The audio codec 108 is part of a host electronic device (not illustrated) such as a mobile telephone or the like. The digital microphone 101 may also be part of the host device and thus the digital microphone may be connected to the codec via some suitable internal connective path. In general it is also known that a peripheral apparatus such as a headset or the like, which may be coupled to the host via some suitable connector such as a jack plug and socket or a USB connector, may comprise a digital microphone that, in use, communicates with the audio codec 108.
The audio codec 108 receives the data signal, DATA, output from digital microphone 101. The data signal DATA may comprise data bits output from the conversion block 107. Conventionally there may be limited signal processing in the digital microphone itself and, as mentioned the output DATA from the digital microphone may typically be an oversampled PDM data stream, although in some instances the modulator 107 may modulate the data to a PCM format.
The audio codec 108 may also control operation of the digital microphone 101. The audio codec 108 may provide a supply or bias voltage VDD to the digital microphone 101. Typically the digital microphone 101 may be controlled to be in a powered-up or powered-down state by the supply voltage VDD. The digital microphone 101 may also have a sleep mode where it is powered by the supply voltage VDD but is effectively inactive.
In use in an active state the audio codec 108 may provide the digital microphone 101 with a clock signal CLK which is used for clocking the conversion block 104, e.g. ADC 106 and coder 107. The clock signal CLK provided by the audio codec 108 thus defines the bit rate of the output DATA signal in use.
As noted above the output (DATA) from the digital microphone 101 may be an oversampled PDM data stream. In some instances an oversampling ratio of around 64, for example, may be used. For a bandwidth of 24 kHz, comparable to a 48 kHz PCM signal, the sample rate for the PDM stream may be about 3.1 MHz. Thus the audio codec 108 may supply the clock signal CLK at around 3.1 MHz in use. Such a frequency of clock signal may be suitable for good quality audio.
In some instances however ultrasonic or near ultrasonic frequencies may be of interest. Ultrasonic detection has been proposed for uses such as proximity detection or gesture recognition where the host device may transmit ultrasonic waves and monitor for any reflection from a nearby object using a digital microphone. It has also been proposed to use ultrasonic or near ultrasonic frequencies as part of device-to-device communications.
In order that such ultrasonic and near-ultrasonic frequencies can be adequately recovered from the data signal DATA outputted by the digital microphone 101, the clock frequency required may be relatively high. For instance in some ultrasonic use cases a clock frequency of 5 MHz or so may be required for a conventional digital microphone.
Generally the higher the clock frequency the more power is consumed by the digital microphone 101 and also the downstream processing and the power required to transmit the digital signal DATA down a physical link. A high clock rate is thus undesirable, especially if the requirement to detect ultrasonic activity may be required for long periods of time even when there may be no activity to detect, e.g. a type of always on functionality listening for any activity.